Cross Section Measurement and Search for New Physics with Top Quark Pair Events Involving a Tau Lepton in ATLAS

In 2011, the ATLAS detector recorded an integrated luminosity of over 5 fb−1 of proton-proton collisions delivered by the LHC at a centre-of-mass √s=7 TeV. The first of two analyses is a test of the standard model and the world’s most precise measurement of the top quark pair production cross section for final states which include a hadronically decaying tau lepton. The second analysis uses the same dataset to search for a charged Higgs boson, also resulting in the world’s best limits for the search channel. In the cross section measurement, 2.1fb−1 of ATLAS proton-proton collision data is used to measure the top quark pair production cross section in events containing an isolated electron or muon and a tau lepton decaying hadronically. After initial event requirements, the leading background comes from top quark pairs with jets faking tau leptons. A fit to a tau lepton identification variable is used to determine the signal yield. The measured cross section 186±13(stat.)+20-19(syst.)±7(lumi.)pb, is in good agreement with the standard model prediction. Several extensions to the standard model predict the existence of at least one charged Higgs boson, H±. According to these extensions, the top quark can decay into a bottom quark and a light charged Higgs boson in addition to the standard model decay to a bottom quark and aW boson. In the second analysis, event yield ratios between different final states are measured using 4.6fb−1 of ATLAS data. This is compared to simulation to search for a violation of lepton universality. This ratio-based method reduces the impact of systematic uncertainties in the analysis. No significant deviations from the standard model predictions are observed. With the assumption that the charged Higgs boson branching ratio to a tau lepton and a neutrino is 100%, upper limits in the range 3.2%–4.4% can be placed on the top quark to charged Higgs branching ratio for 90≤mH±≤140 GeV. After combination with results from a search for charged Higgs bosons in t¯t decays using the thad+jets final state, upper limits on this branching ratio can be set in the range 0.8%–3.4%, for 90≤mH±≤140 GeV.